<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>Title</title>
    <style>
      .container {
        position: relative;
        width: 300px;
        height: 300px;
        background-color: pink;
        margin: 0 auto;
      }
      .container span {
        position: absolute;
        right: 15px;
        top: -10px;
        width: 0;
        height: 0;
        line-height: 0;
        font-size: 0;
        border: 5px solid red;
        border-top: 5px solid transparent;
        border-left: 5px solid transparent;
        border-right: 5px solid transparent;
      }
    </style>
  </head>
  <body>
    <div class="container">
      <span></span>
    </div>
  </body>
</html>
